Disorder-driven quantum phase transitions in two-dimensional altermagnets: Emergence of a marginal metal phase

We study localization properties in disordered two-dimensional altermagnets, an emerging class of material s characterized by momentum-dependent alternating spin polarization and zero net magnetization, in the presence of Rashba spin-orbit coupling. Through large-scale numerical calculations of localization length and finite-size conductance, we map out the phase diagram and identify two distinct disorder-driven transitions: a diffusive metal-insulator transition and a marginal metal-insulator transition. Finite-size scaling analysis reveals that the critical behavior of the diffusive metal-insulator transition deviates from the conventional Anderson scenario, with the critical exponent showing strong sensitivity to the strength of the altermagnetic interaction. Meanwhile, the marginal metal-insulator transition exhibits key signatures of the Berezinskii-Kosterlitz-Thouless (BKT) universality class. Our results highlight the interplay of disorder, spin-orbit coupling, and spin-dependent band topology in altermagnets, and demonstrate that these materials are a promising platform for exploring unconventional quantum phase transitions without net magnetization or an anomalous Hall effect.